1. Field of the Invention
The present invention relates in general to a method for graving printing forms and in particular the invention relates to a method for graving printing forms for rotary printing machines in which rastered masters are employed for scanning.
2. Description of the Prior Art
The use of a rastered original for the graving of a rotary printing form has been disclosed in U.S. Pat. Nos. 3,582,549; 2,543,393 and 3,024,111 and in British Patent Nos. 706,688 and 706,689. These patents disclose methods for producing the printing form in a chemical manner.
There are also electro-mechanical methods known for producing printing forms which comprise the following steps:
(a) opto-electronically scanning the original by means of an optical illumination and scanning system which includes means for placing the original into focus;
(b) conversion of the light signals obtained during scanning of the original into electrical signals which reproduce the intensity of the light signal and then processing the electrical signals in an electronic computer;
(c) the graving of the printing form with a graving tool which is controlled by the electrical signals thus produced.
This method has long been employed in the graving machines of the "Helio-Klischograph" of the assignee of the present application. Unrastered color separation, also called Opals, which have been specifically transferred to opaque, reflecting material are scanned. The production of said Opals is expensive and involved a great deal of time and it is thus desirable to avoid production of opals.
Instead of scanning the opals, the unrastered color separations, it would be much simpler to scan color separations which have already been rastered as is done in offset printing or other printing methods in which rastered color separations are utilized. Such color separations can be produced much cheaper than the opals which have been used up to this time and exist if the original has already been duplicated as, for example, during offset printing. If, however, one uses the rastered color separations in the prior art method instead of the unrastered originals then a moire effect which results in unacceptable printing occurs and the moire results from the superimposition of the raster of the scanned original sheet with the raster with which the graving tool graves the printing form.